Permanent magnet field generators



Och 1955 w. KOBER 2,719,931

PERMANENT MAGNET FIELD GENERATORS Filed March 17, 1951 4 Sheets-Sheet lFig.1.

/- 4 INVENTOR.

A TTORNEYI Oct. 4, 1955 IN V EN TOR. BY W/fl/am Kober ATTORNEYS I N V ENTOR. W////'a/72 Haber BY A TTORNEYS United States Patent PERMANENTMAGNET FIELD GENERATQRS William Koloer, Asbury Park, N. J.

Application March 17, 1951, Serial No. 216,185

Claims. (Cl. 310--156) The object of the invention is to obtain a typeof rotor construction for use in rotating permanent magnet fieldgenerators having a number of special advantages.

One advantage is that a very strong structure, suitable for very highspeeds of rotation is produced.

Another is that wrought, forged or other special mechanical or heattreated material may be used as the main mechanical support and also asthe principal element of the conducting shield for the magnets.

Another advantage is that no casting need be made around the magnets,with possible danger of heat damage to the magnet material.

Another advantage is that very strong materials not producible bycasting may be used in the mechanical structure and conducting shield.

Another advantage is that necessary by-pass magnetic circuits requiredfor proper magnetic structure design are more readily constructed andhave reduced weight.

Another advantage is that short-circuit leakage paths have a greaterfree space to develop, thus reducing the magnetic shock on shortcircuit.

Another advantage is that the magneto-motive force per pole, whichdepends on the length of the magnets, can be chosen for optimum valuewithout interference with other design factors.

These and other advantages are described in the following specificationsand drawings. In the drawings, Figs. 1-2 represent a conventional orradial air gap permanent magnet generator for purposes of comparison.Figs. 3-15, inclusive, show embodiments of the invention,

For a number of well known reasons, the usual structure of a motor orgenerator makes use of a rotor and stator in which the magnetic fieldflux flows from the cylindrical surface of the rotor to the facingcylindrical surface of the stator across a very small air gap. Thisconstruction makes it possible to build up the armature and fieldstructures out of punched sheets, the number of sheets and the resultingthickness of the stack being at the choice of the designer to vary thecapacity of the generator. This punched sheet construction is quiteinexpensive. The armature and field windings are conveniently placed inslots in the stacked punchings. For an electromagnet type of generator,this construction combines the advantages of cheapness of constructionand assembly, strength, and flexibility in varying capacity.

The same structure has heretofore been used with permanent magnet fieldgenerators, using a rotating field in which the flux flows across acylindrical air gap. In such a structure, particularly when the numberof poles is low, such as 2, 4 or 6, it is difiicult to produce propermagnet length and protecting structures without disturbing an efficientrelation between pole pitch and axial length. Since the magnets,protecting structures, and pole pieces tend to slide out of surroundingstructures in an outward radial direction, it is necessary to usefastening devices of sufiicient strength to hold them in place againstinertial forces. Figs. 1 and 2 show a construction of this type for a 4pole permanent magnet field generator, in which the housing and bearingsare not shown. Here, 1 is the stator or armature (winding not shown), 2the field pole faces, 6 is a square soft steel magnetic circuit formagnets 4, parts 5 are non-magnetic spacers between pole faces 2, and 3is cast aluminum which fills the remaining spaces to provide a currentconducting path to assist in shielding the magnets and also to helpsupport the structure.

In the invention, a special structure adapted to a flat fact or axialair gap is used; this structure being uniquely advantageous forpermanent magnet rotating field design.

In Figs. 3, 4, 6 one form of the invention is shown. In this embodiment,it will be seen that the rotor pole faces 8 lie on a flat surface,facing the flat working surface of the stator 7. The constructionsavailable for the stator, and the winding therein, shown in the drawingat 22, will require no further explanation to those skilled in the art.This air gap plan permits a structure of the rotor now to be describedwhich is uniquely suitable in many ways to the special needs ofpermanent magnet field design.

As is well known in permanent magnet generator design, the magnets 9must be looped or encircled by a path of very high conductivity, whichhas a special function in protecting the magnetic state of the magnetsfrom overload and short circuit armature reaction. In this invention,this function is efficiently served by the wrought Duralumin block 10,which has holes to receive the cylindrical magnets 9. The conductingpath is indicated at 11. This structure not only supplies this essentialfunction, but also acts as the principal mechanical structure of therotor. The cross-section outside the magnets at one and the same timefunctions as a heavy-section low resistance conductor and aheavy-section high strength ring retaining the inertial load of themagnets and of its own mass. The sections directly between magnets alsoact as conductors and further mechanical supports. It is well known thatAlnico V and other high performance magnet materials are very weakmechanically, and can carry no significant mechanical loads except indirect compression. It will be noted that in the construction of theinvention the magnet is fully supported, and carries even its own weightentirely in compression. The proper support is enhanced if the magnetshave their cylindrical surfaces accurately finished, and are pressed orshrunk into the holes in the main structure.

The pole piece structure may take the form shown in Fig. 5. Here, itcomprises a composite disc or plate, containing pole faces 8, damperwindings 15, and appropriate non-magnetic metal spacers 16, which mayreplace or share in the mechanical and electrical function of parts 15.The whole structure provides the action of the pole pieces in relationto the stator, and also has the important function of furnishing a fluxpath for the magnets when the generator is disassembled and the rotor isremoved from proximity to the stator. This structure can be welded orbrazed into a self-supporting mechanical whole, centered by the shaftand held in place against the magnets by assembly screws or studs 12,Fig. 3. The load on these studs is a nominal assembly force, not in anypart the very great inertial forces developed at high speed. If desired,the pole face structure can alternatively be non-welded or onlypartially self-supporting, and be wholly or additionally supported by aflange 13 on the main aluminum structure.

To act as a return circuit at the far ends of the magnets, asoft-magnetic steel plate 14 faces against the ends of the magnets. Thisplate will in some designs not be fiat on its outer face, or necessarilyround in periphery but of suitable shape and thickness to conduct theflux with a minimum of total weight. The plate is obviously suitable tocarry its own inertial load, and is held against the magnets by theassembly studs or screws 12, in a manner similar to the pole pieces. Thestuds are preferably of strong non-magnetic steel, such as 18-8stainless or high manganese steel.

The dimensions of the air gap area per pole are determined by well knowndesign factors. Since this shape is in no way related to the length ofthe magnets, it becomes independently possible to choose the magnetlength for optimum design performance.

The main aluminum structure may be made up of two or more disc shapedplates, placed end to end, if the material is not available in greatthicknesses, or if fabrication in such shapes is more convenient. Theuse of several parts does not in any way weaken the mechanical strengthor reduce the conductivity in the essential circling current path.

Although wrought Duralumin has been shown for this structure, anymaterial combining high strength and high conductivity is also suitable.This comprises other aluminum and magnesium alloys, and beryllium-copperand other copper alloys. Alternatively, a part of the strength of themain structure may lie in a wire wound binding 17 around the cylindricalsurface as shown in Figs. 7, 8 or in an outer tube or shell 18, Figs. 9,10, preferably of non-magnetic high strength steel. In this case, theinner material can be made smaller in section, or alternatively chosenof high conductivity material of low weight and only moderate strength,such as pure aluminum, high aluminum content alloys, magnesium alloys,etc.

The adaptability of the pole piece structure to form a proper by-passpath for flux when the rotor is removed from the assembly is obviousfrom the fact that the available radial depth is much greater than thethickness of the magnet, that the axial length of the magnet and henceits M. M. F. is at the choice of the designer without interfering withother design factors, and that projection of the pole piece along thecylindrical surface of the main structure is also possible, as shown inFig. 11. Such by-pass paths have another vital function. As is wellknown, when the generator is running and a short circuit takes place, itis necessary for an amount of fiux that usually exceeds the normalworking flux to escape from the pole pieces. Some of this fluxorigimates in the stator and is forced into the pole pieces across theair gap, and in general the stator is not under these conditions anypath for the field flux. Also, the magnet structure must be aifected aslittle as possible, and hence should continue its normal flux output.Thus, the pole pieces must serve as an escape path for large amounts offlux without entry into the stator or magnets. To do this, very greatmagneto forces are called into play, since the available path is alwayslimited.

It is the main function of the electrical conductivity built into paths11, Fig. 6 of the main structure to supply this very greatmagneto-motive force. Obviously, this force is reduced when the escapepath is more permeable, and this permits a reduction in conductingsection, with resulting saving in size and weight, or alternatively, ahigher state of magnetization of the magnet.

A study of Fig. 11 will reveal that 180 degrees in the plane of thepaper and the full cylindrical surface outside the pole pieces isavailable space for permitting this flux 20 to go into the air. Alsoavailable are the by-pass paths across the non-magnetic inserts betweenadjacent pole pieces, and flux inward in the shaft section. Thepreviously mentioned extension 19 of the pole piece back over thecylindrical surface of the main structure as shown in Fig. 11 servesfurther to increase the surface and space available to dissipate thisflux. It will be appreciated that the essential geometry of thestructure of the invention supplies a roomier and hence more permeablepath than that available in the old cylindrical air gap design. Theimportance of this escape path cannot be exaggerated in the theory ofprotection of the magnets on which modern permanent magnet builtgenerators operate.

The magnet cross-section shown in previous embodiments is round. This isconvenient in permitting drilling of the main structure, and easygrinding of the magnets. However, when maximum magnet section withminimum diameter is desired, a shape making better use of the spaceavailable, such as the sector shown in Fig. 12 is obviouslyadvantageous. Each magnet 9 can be built up of triangular, square, orother compactly fitted rods in cases where this is advantageous.

in the figures, a four pole generator has been shown. The structure,however, is adapted to any number of poles. Figs. 13, 14 show the rotorof an 18 pole generator, using magnets 9 of square cross section. Here,openings 21 in structure 10 serve to lighten the Weight of the unit.Other parts have functions obviously similar to those before described.

Fig. 15 shows a 2 pole rotor according to the principles of theinvention. Here magnets 9 are made up of a number of sectors, since toolarge a mass of magnet material cannot be properly heat treatedthroughout to develop its full potential characteristics.

I claim:

1. In an axial air gap generator having a rotating field comprisingpermanent magnets, said permanent magnets requiring a flux conductingpath when said rotating field is removed from said generator, meanssupporting said magnets against centrifugal forces produced by rotationof said field, pole pieces in contact with said magnets, said polepieces having their flux producing faces lying substantially in a singleplane and having outer cylindrical appendages extending exteriorly ofsaid magnet supporting means, whereby a maximum volume of free space isavailable for air conduction of said flux, and a supporting structurebearing against the outer edge portion of said pole pieces to restrainthe same against centrifugal forces produced by rotation of said field.

2. In a generator having a rotating field comprising permanent magnets,a stator, and an axial air gap therebetween, said permanent magnetsrequiring a flux conducting path independent of said stator when anoverload or short circuit is applied to said generator, a block ofmaterial of high conductivity and high strength encircling said magnets,whereby to support said magnets against centrifugal forces produced byrotation of said field and simultaneously provide a highly conductivepath encircling said magnets for protecting the magnetic state thereof,pole pieces at one end of said block of material in contact with saidmagnets, and means supporting said pole pieces against centrifugalforces produced by rotation of said field, said pole pieces having theirflux producing faces lying substantially in a single plane and havingouter cylindrical appendages extending along the outer surface of saidblock of material, whereby a maximum volume of free space outside ofsaid stator and rotor is available for air conduction of said flux.

3. In an axial air gap generator having a rotating permanent magnetfield structure and a stator, a field structure comprising, a rotor bodyformed of a material characterized by a high degree of electricalconductivity and a high degree of mechanical strength, said rotor bodybeing formed with a series of magnet receiving apertures extendingtherethrough in substantial parallelism with the axis of rotation ofsaid field structure, permanent magnets fitted in said apertures withthe ends of said magnets being free in said apertures for the entranceand exit of magnetic flux, said rotor body surrounding and enclosingsaid magnets for substantially the full length thereof whereby saidrotor body provides a strong mechanical support for said magnets alongsubstantially the complete length thereof to permit high speeds ofrotation of said field structure and simultaneously provides a highlyelectrically conductive path independent of said stator encircling saidmagnets for substantially the complete length thereof to protect themagnetic state thereof from overload and short circuit armaturereaction,pole p'iec'e means arranged in contact with the end faces of saidmagnets adjacent said stator, said pole piece means having fluxproducing faces lying substantially in a single plane whereby a maximumvolume of free space outside of said stator and said rotor body isavailable for air conduction of flux, and flux conducting means incontact with the opposite end faces of said magnets.

4. In an axial air gap generator, a rotating permanent magnet fieldstructure comprising, a rotor body formed of a material characterized bya high degree of electrical conductivity and a high degree of mechanicalstrength, said rotor body having a series of magnet receiving aperturesextending therethrough in substantial parallelism with the axis ofrotation of said field structure, permanent magnets fitted in saidapertures with the ends of said magnets being free in said apertures forthe entrance and exit of magnetic flux, said rotor body extendingsubstantially the full length of said magnets to substantiallycompletely enclose the same, said rotor body thereby providing a strongmechanical support for said magnets along substantially the full lengththereof to permit high speeds of rotation of said field structure andsimultaneously providing a highly electrically conductive pathsubstantially completely enclosing said magnets for protecting themagnetic state thereof from overload and short circuit armaturereaction, pole piece means carried by said rotor body on the air gap endface thereof in contact with the corresponding end faces of saidmagnets, means including a flange formed integrally with said rotor bodyand encircling said pole piece means to restrain the same againstcentrifugal force produced by rotation of said field structure, and fiuxconducting structures arranged in contact with the opposite end faces ofsaid magnets.

5. In an axial air gap generator, a rotating permanent magnet fieldstructure, a stator, said field structure comprising, a rotor bodyformed of a material characterized by a high degree of electricalconductivity and a high degree of mechanical strength, said rotor bodyhaving a series of magnet receiving apertures extending therethrough insubstantial parallelism with the axis of rotation of said fieldstructure, permanent magnets fitted in said apertures with the ends ofsaid magnets being free in said apertures for the entrance and exit ofmagnetic flux, said rotor body completely surrounding said magnets forsubstantially the full length thereof, said rotor body thereby providinga strong mechanical support completely supporting said magnets along thelength thereof against centrifugal force to permit high speeds ofrotation of said field structure and simultaneously encircling saidmagnets with a highly electrically conductive path for protecting themagnetic state thereof from overload and short circuit armaturereaction, pole piece means arranged in contact with the end faces ofsaid magnets adjacent said stator, and flux conducting means in contactwith the opposite end force of said magnets.

6. In an axial air gap generator, a rotating permanent magnet fieldstructure comprising, a rotor body formed of a material characterized bya high degree of electrical conductivity and a high degree of mechanicalstrength, said rotor body having a series of magnet-receiving aperturesextending therethrough in substantial parallelism with the axis ofrotation of said field structure, permanent magnets fitted in saidapertures with the ends of said magnets being free in said apertures forthe entrance and exit of magnetic flux, said rotor body extendingsubstantially the full length of said magnets to substantiallycompletely encase the same, said rotor body thereby providing a strongmechanical support for said magnets along substantially the full lengththereof to permit high speeds of rotation of said field structure andsimultaneously providing a highly electrically conductive pathsubstantially completely enclosing said magnets for protecting themagnetic state thereof from overload and short circuit armaturereaction, and a pole piece structure arranged on the air gap end face ofsaid rotor body, said pole piece structure including pole pieces incontact with the air gap end faces of said magnets and plate meanssecured to said rotor body end face and having cut out portionsaccommodating said pole pieces therein, said plate means includingnon-magnetic spacing material between said pole pieces. I

7. In an axial air gap generator, a rotating permanen magnet fieldstructure, a stator, said field structure comprising, a rotor bodyformed of a wrought material characterized by a high degree ofelectrical conductivity and a high degree of mechanical strength, saidrotor body having a series of magnet receiving apertures extendingtherethrough in substantial parallelism with the axis of rotation ofsaid field structure, permanent magnets fitted in said apertures withthe ends of said magnets being free in said apertures for the entranceand exit of magnetic flux, said rotor body completely surrounding saidmagnets for substantially the full length thereof, said rotor bodythereby providing a strong mechanical support completely supporting saidmagnets along the length thereof against centrifugal force to permithigh speeds of rotation of said field structure and simultaneouslyencircling said magnets with a highly electrically conductive path forprotecting the magnetic state thereof from overload and short circuitarmature reaction, pole piece means arranged in contact with the endfaces of said magnets adjacent said stator, and flux conducting means incontact with the opposite end faces of said magnets.

8. In an axial air gap generator, a rotating permanent magnet fieldstructure, a stator, said field structure comprising, a rotor bodyformed of Duralumin characterized by a high degree of electricalconductivity and a high degree of mechanical strength, said rotor bodyhaving a series of magnet receiving apertures extending therethrough insubstantial parallelism with the axis of rotation of said fieldstructure, permanent magnets fitted in said apertures with the ends ofsaid magnets being free in said apertures for the entrance and exit ofmagnetic flux, said rotor body completely surrounding said magnets forsubstantially the full length thereof, said rotor body thereby providinga strong mechanical support completely supporting said magnets along thelength thereof against centrifugal force to permit high speeds ofrotation of said field structure and simultaneously encircling saidmagnets with a highly electrically conductive path for protecting themagnetic state thereof from overload and short circuit armaturereaction, pole piece means arranged in contact with the end faces ofsaid magnets adjacent said stator, and flux conducting means in contactwith the opposite end faces of said magnets.

9. In an axial air gap generator, a rotating permanent magnet fieldstructure, a stator, said field structure comprising, a rotor bodyformed of an alloy of a material selected from the group consisting ofaluminum, magnesium and copper characterized by a high degree ofelectrical conductivity and a high degree of mechanical strength, saidrotor body having a series of magnet receiving apertures extendingtherethrough in substantial parallelism with the axis of rotation ofsaid field structure, permanent magnets fitted in said apertures withthe ends of said magnets being free in said apertures for the entranceand exit of magnetic flux, said rotor body completely surrounding saidmagnets for substantially the full length thereof, said rotor bodythereby providing a strong mechanical support completely supporting saidmagnets along the length thereof against centrifugal force to permithigh speeds of rotation of said field structure and simultaneouslyencircling said magnets with a highly electrically conductive path forprotecting the magnetic state thereof from overload and short circuitarmature reaction, pole piece means arranged in contact with the endfaces of said magnets adjacent said stator, and flux conducting means incontact with the opposite end faces of said magnets.

10. In an axial air gap generator, a rotating permanent magnet fieldstructure, a stator, said field structure comprising, a rotor bodyformed of a material characterized by a high degree of electricalconductivity and a high degree of mechanical strength, said rotor bodyhaving a series of magnet receiving apertures extending therethrough insubstantial parallelism with the axis of rotation of said fieldstructure, permanent magnets fitted in said apertures with the ends ofsaid magnets being free in said apertures for the entrance and exit ofmagnetic flux, said rotor body completely surrounding said magnets forsubstantially the full length thereof, said rotor body thereby providinga strong mechanical support completely supporting said magnets along thelength thereof against centrifugal force to permit high speeds ofrotation of said field structure and simultaneously encircling saidmagnets with a highly electrically conductive path for protecting themagnetic state thereof from overload and short circuit armaturereaction, pole piece means arranged in contact with the end faces ofsaid magnets adjacent said stator, flux conducting means in contact Withthe opposite end faces of said magnets, and reinforcing meanssurrounding said rotor body.

References. Cited in the file of this patent UNITED STATES PATENTS271,979 Gordon Feb. 6, 1883 496,514 Frische May 2, 1893 1,359,333 CowlesNov. 16, 1920 2,059,518 Harley Nov. 31, 1936 2,475,776 Brainard July 12,1949 2,488,437 Schaefer Nov. 15, 1949 2,493,102 Brainard Jan. 3, 1950FOREIGN PATENTS 288,232 Great Britain Jan. 17, 1929 321,549 GreatBritain Nov. 14, 1929 516,221 Germany Jan. 20, 1931 235,423 SwitzerlandApr. 3, 1945

